Oxide catalyst composition

ABSTRACT

An oxide catalyst composition for use in producing methacrolein or a mixture of methacrolein and methacrylic acid, wherein the oxide catalyst composition is represented by the formula (Mo+W) l2 Bi a A b B c Fe d X e Sb f O g , wherein: A is at least one member selected from the group consisting of Y and the elements of the lanthanoid series exclusive of Pm; B is at least one member selected from the group consisting of K, Rb and Cs; X is Co solely, or a mixture of Co and at least one member selected from the group consisting of Mg and Ni; and a, b, c, d, e, f and g are, respectively, the atomic ratios of Bi, A, B, Fe, X, Sb and O, relative to twelve atoms of the total of Mo and W, wherein the atomic ratios (a to f) of the elements and the relationship between the amounts of the elements are chosen so as to satisfy specific requirements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxide catalyst composition. Moreparticularly, the present invention is concerned with an oxide catalystcomposition for use in producing methacrolein or a mixture ofmethacrolein and methacrylic acid by reacting at least one memberselected from the group consisting of isobutylene and t-butanol with amolecular oxygen-containing gas, wherein the oxide catalyst compositioncomprises, in specific ratios, molybdenum or a mixture of molybdenum andtungsten; bismuth; iron; antimony; at least one member selected from thegroup consisting of yttrium and the elements of the lanthanoid seriesexclusive of promethium; and at least one member selected from the groupconsisting of potassium, rubidium and cesium; and cobalt solely, or amixture of cobalt and at least one member selected from the groupconsisting of magnesium and nickel.

The oxide catalyst composition of the present invention exhibits notonly a prolonged catalyst life due to its excellent properties withrespect to thermal stability and reduction resistance, but alsoexcellent selectivity for the desired product. By the use of the oxidecatalyst composition of the present invention for producing methacroleinor a mixture of methacrolein and methacrylic acid, it becomes possibleto stably produce the desired product for a long time while holding downthe amount of by-produced impurities, e.g. diacetyl. The producedmethacrolein or mixture of methacrolein and methacrylic acid has lowcontents of the by-produced impurities, e.g. diacetyl, and suchmethacrolein or mixture of methacrolein and methacrylic acid is veryadvantageous as a raw material for producing methyl methacrylate havingexcellent transparency. A methyl methacrylate polymer having excellenttransparency, which can be obtained by polymerizing such highlytransparent methyl methacrylate monomer, can be advantageously used as asubstitute for glass and quartz in application fields requiring hightransparency, such as optical fibers, light guide plates and the like;thus, such highly transparent methyl methacrylate polymer has very highcommercial value.

2. Prior Art

A polymer produced from methyl methacrylate is characterized in that itis glassy, hard and transparent, and such polymer is frequently used asa substitute for glass. In recent years, in the fields related tooptical fibers, a methyl methacrylate polymer is attracting attention asan optical material which can substitute for quartz, and the use of themethyl methacrylate polymer is spreading. Therefore, the methylmethacrylate polymer used as a substitute for glass or quartz isrequired to have high transparency and high weathering resistance. Forobtaining such an excellent methyl methacrylate polymer, it is importantthat, in a methyl methacrylate monomer used as the raw materialtherefor, the amounts of trace impurities be very small which lower thetransparency and weathering resistance of the methyl methacrylatepolymer.

As methods for producing methyl methacrylate, which is a compound highlyuseful in the industry, there are known two methods: a “direct ML-to-MMAprocess” comprising two reaction steps and a “via methacrylic acidprocess” comprising three reaction steps. The direct ML-to-MMA processcomprises two catalytic reaction steps, wherein the first reaction stepcomprises subjecting isobutylene and/or t-butanol as a starting materialto a gaseous phase catalytic oxidation reaction with a molecularoxygen-containing gas in the presence of an oxide catalyst (hereinafter,this catalyst is frequently referred to as a “first stage catalyst”) tothereby obtain methacrolein, and the second reaction step comprisessubjecting the obtained methacrolein to a gaseous phase catalyticreaction with methanol and a molecular oxygen-containing gas in thepresence of a carrier-supported catalyst containing palladium(hereinafter, this catalyst is frequently referred to as a “second stagecatalyst”), to thereby produce methyl methacrylate (MMA) by one stepdirectly from methacrolein (ML).

In the recent studies by the present inventors on the direct ML-to-MMAprocess, it has been found that substances which show an absorption inthe visible light range of 400 nm to 780 nm, are causative ofdiscoloration of methyl methacrylate. The substances showing anabsorption in the visible light range include not only diacetyl, whichis conventionally known as being causative of discoloration, but alsopyruvic aldehyde, 2-acetylfuran and the like. Thus, pyruvic aldehyde,2-acetylfuran and the like have been found to be substances causative ofthe discoloration of methyl methacrylate.

The first stage catalyst used in the direct ML-to-MMA process (whereinthe first stage catalyst is used for producing methacrolein bysubjecting at least one member selected from the group consisting ofisobutylene and t-butanol to a gaseous phase catalytic oxidationreaction with a molecular oxygen-containing gas) was proposed by thepresent inventors (see, for example, International Patent ApplicationPublication No. WO 95/35273 and Unexamined Japanese Patent ApplicationLaid-Open Specification No. Hei 10-216523). However, at the time of thedevelopment of the catalyst, it was not well recognized that theimpurities, e.g. diacetyl, which are by-produced by the catalyst arecausative of the discoloration of methyl methacrylate.

U.S. Pat. No. 4,249,019, Japanese Patent Application Publication No. Sho57-35859, U.S. Pat. No. 4,518,796 and the like propose various types ofpalladium-containing second stage catalyst for use in the secondreaction step of the direct ML-to-MMA process (wherein the second stagecatalyst is used for producing methyl methacrylate by subjectingmethacrolein to a gaseous phase catalytic reaction with methanol and amolecular oxygen-containing gas). In the studies of the presentinventors, it has been found that, since the second stage catalyticreaction (for producing methyl methacrylate by reacting methacrolein,methanol and a molecular oxygen-containing gas) is conducted undermoderate reaction conditions wherein the reaction temperature is in therange of from room temperature to 100° C., the impurities, e.g.diacetyl, which would be causative of the discoloration of the resultantmethyl methacrylate remain almost unreacted and undecomposed during thesecond stage catalytic reaction, and the impurities as such are carriedover into a subsequent purification step. When the impurities causativeof discoloration are carried over into the purification step, it isnecessary to perform repeatedly the purification operation for removingthe impurities from the methyl methacrylate. As a result, greatcommercial disadvantages are posed in that the repeated purificationoperation causes a loss of methyl methacrylate, leading to an increasein the production cost.

The via methacrylic acid process is also a method for producing methylmethacrylate by using isobutylene and/or t-butanol as a startingmaterial. The via methacrylic acid process is described at pages 172 to176 of “Sekiyu Kagaku Purosesu (Petrochemical Process)”, published byKodansha Scientific, Inc., Japan. The document states that the viamethacrylic acid process comprises three reaction steps, that is, afirst oxidation step, a second oxidation step, and an esterificationstep. The first oxidation step is a step of subjecting at least onestarting material selected from the group consisting of isobutylene andt-butanol to a gaseous phase catalytic oxidation reaction with molecularoxygen in the presence of a catalyst, to thereby obtain methacrolein.The second oxidation step is a step of subjecting the methacroleinobtained in the first oxidation step to a gaseous phase catalyticoxidation reaction with molecular oxygen in the presence of a catalyst,to thereby obtain methacrylic acid. The esterification step is a step ofsubjecting the methacrylic acid obtained in the second oxidation step toesterification, to thereby obtain methyl methacrylate.

Various proposals have been made on the catalyst used in the firstoxidation step of the via methacrylic acid process, that is, thecatalyst used for producing methacrolein by subjecting at least onestarting material selected from the group consisting of isobutylene andt-butanol to a gaseous phase catalytic oxidation reaction. Most of suchproposals are concerned with the selection of the types and ratios ofthe components of the catalyst. For example, there can be mentionedExamined Japanese Patent Application Publication No. Sho 48-17253(corresponding to Canadian Patent No. 947,772), U.S. Pat. Nos. 4,001,317and 4,537,874, and Unexamined Japanese Patent Application Laid-OpenSpecification Nos. Sho 60-163830, Sho 63-122641 and Hei 2-227140. Thecatalysts disclosed in these patent documents are aimed mainly atachieving an improved yield of the desired product; and, in these patentdocuments, the experimental data concerning the performance of thecatalysts are only the data of the conversion of isobutylene andt-butanol and the data of the yield of and selectivity for methacroleinor methacrylic acid.

With respect to the by-produced impurities formed in the via methacrylicacid process (that is, the products other than methacrolein andmethacrylic acid which are, respectively, the desired products of thefirst and second oxidation steps of the via methacrylic acid process),the descriptions of prior art documents are as follows. For example,Examined Japanese Patent Application Publication No. Sho 53-23809describes the selectivities for acetic acid, CO₂ and CO; ExaminedJapanese Patent Application Publication No. Sho 57-61011 describes theselectivities for acetone and acetic acid; and Examined Japanese PatentApplication Publication Nos. Sho 51-13125 and Sho 51-12605 each describethe selectivities for CO₂ and CO. In addition, U.S. Pat. No. 3,928,462(corresponding to Examined Japanese Patent Application Publication Nos.Sho 47-32043 and Sho 47-32044) describes that the selectivity foracrolein is 5% to 6%. All of the impurities described in theabove-mentioned patent documents are different from the substances whichare causative of discoloration.

In addition, Examined Japanese Patent Application Publication No. Hei5-86939 describes that a gaseous, oxidation reaction product, which isobtained by subjecting at least one member selected from the groupconsisting of isobutylene and t-butanol to a gaseous phase catalyticoxidation for producing methacrolein, contains not only methacrolein andmethacrylic acid, but also low boiling point by-products (such asacetoaldehyde, acetone, acrolein, acetic acid and acrylic acid), highboiling point by-products (such as maleic acid and aromatic carboxylicacids), polymeric substances and tarry substances. For obtaining agaseous, oxidation reaction product containing substantially nopolymeric substances and the like, the above-mentioned patent documentproposes a method in which the gaseous, oxidation reaction product iscontacted with a solid alkaline earth metal compound to thereby suppressthe formation of the polymeric substances and the like, and alsoproposes a method in which the polymeric substances and the likecontained in the gaseous, oxidation reaction product are decomposed andremoved therefrom. This patent document describes the amounts of theby-produced maleic acid and the by-produced polymeric substances, inaddition to the amounts of the produced methacrolein and the producedmethacrylic acid. However, this patent document has no description aboutthe trace impurities which are causative of the discoloration of methylmethacrylate.

In addition, it is considered that, since a high reaction temperature,namely 300° C. to 400° C., is used in the second oxidation step of thevia methacrylic acid process, most of the substances (such as diacetyl)causative of discoloration which are by-produced in the first oxidationstep are decomposed during the second oxidation step of the viamethacrylic acid process. Consequently, the substances causative ofdiscoloration have not been particularly considered as being a problemin the case of the via methacrylic acid process. However, since not allof the substances causative of discoloration are decomposed in thesecond oxidation step of the via methacrylic acid process, it isnecessary that a catalyst which does not by-produce the substancescausative of discoloration be used in the first oxidation step of thevia methacrylic acid process.

In connection with the first and second oxidation steps of the viamethacrylic acid process, Japanese Patent No. 2509049 (corresponding toU.S. Pat. No. 5,264,627) describes a measure for reducing the amounts ofthe impurities which are causative of the discoloration of methylmethacrylate. In this patent document, the via methacrylic acid processis performed as follows. At least one compound selected from the groupconsisting of isobutylene, t-butanol and methyl-t-butyl ether isintroduced, together with molecular oxygen, into a shell-and-tube heatexchanger type first oxidation reactor packed with an oxide catalystcontaining bismuth, molybdenum and iron, and a gaseous phase catalyticoxidation reaction is effected therein to thereby obtain a gaseousreaction product comprised mainly of methacrolein. Subsequently, themethacrolein-containing gaseous reaction product and molecular oxygenare introduced into a shell-and-tube heat exchanger type second reactorpacked with an oxide catalyst containing molybdenum and phosphorus, anda gaseous phase catalytic oxidation reaction is effected therein tothereby produce a gaseous reaction product comprised mainly ofmethacrylic acid. In this method, the space of the gas outlet portion ofthe second reactor is packed with a solid packing so as to reduce thevolume of the space (of the gas outlet portion) which is presentdownstream of the catalyst bed in the second reactor, wherein thereduction of the volume of the space of the gas outlet portion isintended to shorten the residence time of the gaseous reaction productin the space of the gas outlet portion, thereby suppressing theby-production of diketones. This patent document further states that,when diketones are contained in methacrylic acid obtained after thefirst and second oxidation steps, a problem arises in that the diketonesare converted into furan compounds in the final methacrylate polymer andthe furan compounds cause discoloration of the polymer. (In the WorkingExamples of this patent document, acetonitrile acetone is mentioned as adiketone.)

Therefore, when the catalyst used in the first reactor used in theabove-described method is improved so as to decrease the amount of theby-produced diketones, the improved catalyst is effective for decreasingthe amounts of the substances causative of the discoloration of methylmethacrylate produced by the via methacrylic acid process.

As can be seen from the descriptions of the above-mentioned patentdocuments, it has been recognized to some extent that the discolorationof methyl methacrylate produced by the via methacrylic acid process iscaused by the impurities, e.g. diacetyl, which are by-produced in thefirst oxidation step of the via methacrylic acid process, namelyby-produced in a reaction in which at least one member selected from thegroup consisting of isobutylene and t-butanol is subjected to a gaseousphase catalytic oxidation reaction in the presence of a catalyst,thereby producing methacrolein. However, there have been no methods forimproving the catalyst used for producing methacrolein in the firstoxidation step of the via methacrylic acid process wherein theimprovement is for decreasing the amounts of the substances (such asdiacetyl) causative of discoloration which are by-produced in the firstoxidation step of the via methacrylic acid process.

When methyl methacrylate is produced by the direct ML-to-MMA process,oxidative methyl esterification (the second stage reaction) is conductedat a low temperature (room temperature to 100° C.). The advantage of thedirect ML-to-MMA process is that the yield of methyl methacrylate ishigher than in the case of the via methacrylic acid process. However,the direct ML-to-MMA process has a problem in that most of thesubstances (such as diacetyl) causative of discoloration are notdecomposed by the second stage reaction catalyst and are carried overinto the subsequent purification step. Therefore, in the directML-to-MMA process, for improving the quality of methyl methacrylate, itis necessary to decrease the amount of the substances causative ofdiscoloration, which are impurities by-produced in the first stagereaction. Thus, there is a strong demand for the development of acatalyst which is advantageous not only in that a high selectivity formethacrolein can be achieved, but also in that the catalyst has highthermal stability and high reduction resistance, and the selectivity forthe impurities causative of discoloration can be held down to a minimumlevel.

SUMMARY OF THE INVENTION

In this situation, the present inventors have made extensive andintensive studies toward developing an oxide catalyst which isadvantageous not only in that a high selectivity for methacrolein can beachieved, but also in that the catalyst has high thermal stability andhigh reduction resistance, and the selectivity for the impuritiescausative of discoloration can be held down to a minimum level. As aresult, it has unexpectedly been found that the above-mentioned objectcan be attained by an oxide catalyst composition which comprisesmolybdenum or a mixture of molybdenum and tungsten; bismuth; iron;antimony; at least one member selected from the group consisting ofyttrium and the elements of the lanthanoid series exclusive ofpromethium; and at least one member selected from the group consistingof potassium, rubidium and cesium; and cobalt solely, or a mixture ofcobalt and at least one member selected from the group consisting ofmagnesium and nickel, wherein the respective atomic ratios of theabove-mentioned elements and the relationship between the amounts of theabove-mentioned elements are chosen so as to satisfy specificrequirements. That is, it has unexpectedly been found that such oxidecatalyst composition exhibits not only a prolonged catalyst life due toits excellent properties with respect to thermal stability and reductionresistance, but also a high selectivity for methacrolein and a lowselectivity for the impurities which are causative of discoloration ofmethyl methacrylate. The present invention has been completed, based onthis novel finding.

Accordingly, it is a primary object of the present invention to providean oxide catalyst composition for use in producing methacrolein or amixture of methacrolein and methacrylic acid wherein the methacrolein ormixture has low impurity content and is very advantageous as a rawmaterial for producing a highly transparent methyl methacrylate.

The foregoing and other objects, features and advantages of the presentinvention will be apparent to those skilled in the art from thefollowing detailed description taken in connection with the accompanyingdrawing and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

FIGURE is a flow chart showing the steps of the direct ML-to-MMA processconducted in the Examples for producing methyl methacrylate.

DESCRIPTION OF REFERENCE NUMERALS

1: First stage reaction step

2: Methacrolein (MAL) absorption step

3: Second stage reaction step

4: Methacrolein (MAL) recovery step

5: Acid treatment/water-oil separation step

6: High boiling point substance separation step

7: Low boiling point substance separation step

8: Methyl methacrylate (MMA) purification step

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided an oxide catalystcomposition for use in producing methacrolein or a mixture ofmethacrolein and methacrylic acid by reacting at least one memberselected from the group consisting of isobutylene and t-butanol with amolecular oxygen-containing gas, the oxide catalyst composition beingrepresented by the following formula (I):(Mo+W)₁₂Bi_(a)A_(b)B_(c)Fe_(d)X_(e)Sb_(f)O_(g)  (I)wherein:

-   -   A is at least one member selected from the group consisting of        lanthanum, cerium, praseodymium, neodymium, samarium, europium,        gadolinium, terbium, dysprosium, holmium, erbium, thulium,        ytterbium, lutetium and yttrium;    -   B is at least one member selected from the group consisting of        potassium, rubidium and cesium;    -   X is cobalt solely, or a mixture of cobalt and at least one        member selected from the group consisting of magnesium and        nickel;    -   wherein the number of molybdenum (Mo) atoms is in the range of        from more than 9 to 12, and the number of tungsten (W) atoms is        in the range of from 0 to less than 3, each relative to twelve        atoms of the total of molybdenum (Mo) and tungsten (W); and    -   a, b, c, d, e, f and g are, respectively, the atomic ratios of        bismuth (Bi), A, B, iron (Fe), X, antimony (Sb) and oxygen (O),        relative to twelve atoms of the total of molybdenum (Mo) and        tungsten (W),    -   wherein        -   0<a≦8,        -   0<b≦8,        -   0<c<3,        -   0.2<d<5,        -   1≦e≦12,        -   0.1<f<3, and        -   g is the number of oxygen atoms required to satisfy the            valence requirements of the other elements present; and    -   wherein a, b, c, d and f satisfy the requirements of the        following formulae:        0.02<b/(a+b+c)<0.6,        0<c/(a+b+c)≦0.9,        0.01<d/(a+b+d)≦0.9, and        0.1<d−f<2.5.

For easy understanding of the present invention, the essential featuresand various preferred embodiments of the present invention areenumerated below.

-   1. An oxide catalyst composition for use in producing methacrolein    or a mixture of methacrolein and methacrylic acid by reacting at    least one member selected from the group consisting of isobutylene    and t-butanol with a molecular oxygen-containing gas, the oxide    catalyst composition being represented by the following formula (I):    (Mo+W)₁₂Bi_(a)A_(b)B_(c)Fe_(d)X_(e)Sb_(f)O_(g)  (I)-   wherein:    -   A is at least one member selected from the group consisting of        lanthanum, cerium, praseodymium, neodymium, samarium, europium,        gadolinium, terbium, dysprosium, holmium, erbium, thulium,        ytterbium, lutetium and yttrium;    -   B is at least one member selected from the group consisting of        potassium, rubidium and cesium;    -   X is cobalt solely, or a mixture of cobalt and at least one        member selected from the group consisting of magnesium and        nickel;    -   wherein the number of molybdenum (Mo) atoms is in the range of        from more than 9 to 12, and the number of tungsten (W) atoms is        in the range of from 0 to less than 3, each relative to twelve        atoms of the total of molybdenum (Mo) and tungsten (W); and    -   a, b, c, d, e, f and g are, respectively, the atomic ratios of        bismuth (Bi), A, B, iron (Fe), X, antimony (Sb) and oxygen (O),        relative to twelve atoms of the total of molybdenum (Mo) and        tungsten (W),    -   wherein        -   0<a≦8,        -   0<b≦8,        -   0<c<3,        -   0.2<d<5,        -   1≦e≦12,        -   0.1<f<3, and        -   g is the number of oxygen atoms required to satisfy the            valence requirements of the other elements present; and    -   wherein a, b, c, d and f satisfy the requirements of the        following formulae:        0.02<b/(a+b+c)<0.6,        0<c/(a+b+c)≦0.9,        0.01<d/(a+b+d)≦0.9, and        0.1<d−f<2.5.-   2. The oxide catalyst composition according to item 1 above,    wherein, in the mixture X in formula (I), the atomic ratio of cobalt    to the total of cobalt, magnesium and nickel is 0.5 or more,

wherein, when the mixture X in formula (I) contains magnesium, theatomic ratio of magnesium to the total of cobalt, magnesium and nickelin the mixture X is 0.5 or less, and

wherein, when the mixture X in formula (I) contains nickel, the atomicratio of nickel to the total of cobalt, magnesium and nickel in themixture X is less than 0.33.

-   3. The oxide catalyst composition according to item 1 or 2 above,    wherein a, b and c in formula (I) satisfy the requirements of the    formula: 0.05<b/(a+b+c)<0.5.-   4. The oxide catalyst composition according to any one of items 1 to    3 above, wherein a, b and c in formula (I) satisfy the requirements    of the formula: 0.1<c/(a+b+c)<0.8.-   5. The oxide catalyst composition according to any one of items 1 to    4 above, wherein a, b, d and f in formula (I) satisfy the    requirements of the formulae:    0.2<d/(a+b+d)<0.9 and 0.3≦d−f≦2.3.

Hereinbelow, the present invention will be described in detail.

The oxide catalyst composition of the present invention is representedby the following formula (I):(Mo+W)₁₂Bi_(a)A_(b)B_(c)Fe_(d)X_(e)Sb_(f)O_(g)  (I)

-   wherein:    -   A is at least one member selected from the group consisting of        lanthanum, cerium, praseodymium, neodymium, samarium, europium,        gadolinium, terbium, dysprosium, holmium, erbium, thulium,        ytterbium, lutetium and yttrium;    -   B is at least one member selected from the group consisting of        potassium, rubidium and cesium;    -   X is cobalt solely, or a mixture of cobalt and at least one        member selected from the group consisting of magnesium and        nickel;    -   wherein the number of molybdenum (Mo) atoms is in the range of        from more than 9 to 12, and the number of tungsten (W) atoms is        in the range of from 0 to less than 3, each relative to twelve        atoms of the total of molybdenum (Mo) and tungsten (W); and    -   a, b, c, d, e, f and g are, respectively, the atomic ratios of        bismuth (Bi), A, B, iron (Fe), X, antimony (Sb) and oxygen (O),        relative to twelve atoms of the total of molybdenum (Mo) and        tungsten (W),    -   wherein        -   0<a≦8,        -   0<b≦8,        -   0<c<3,        -   0.2<d<5,        -   1≦e≦12,        -   0.1<f<3, and        -   g is the number of oxygen atoms required to satisfy the            valence requirements of the other elements present, and    -   wherein a, b, c, d and f satisfy the requirements of the        following formulae:        0.02<b/(a+b+c)<0.6,        0<c/(a+b+c)≦0.9,        0.01<d/(a+b+d)≦0.9, and        0.1<d−f<2.5.

In the present invention, molybdenum (Mo) is an indispensable componentfor the oxide catalyst composition, but tungsten (W) may be used inpartial substitution for molybdenum. The number of molybdenum atoms isin the range of from more than 9 to 12, preferably from more than 9.5 to12, and the number of tungsten atoms is in the range of from 0 to lessthan 3, preferably from 0 to less than 2.5, each relative to twelveatoms of the total of molybdenum and tungsten.

In the present invention, bismuth (Bi) is indispensable for thesynthesis of methacrolein. For providing an oxide catalyst compositioncapable of achieving the catalytic performances aimed at in the presentinvention, it is necessary that the atomic ratio (a) of bismuth,relative to twelve atoms of the total of molybdenum and tungsten,satisfy the relationship 0<a≦8.

A in formula (I) is at least one member selected from the groupconsisting of yttrium and the elements of the lanthanoid seriesexclusive of promethium, that is, at least one member selected from thegroup consisting of lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), lutetium (Lu) and yttrium (Y). Element A isindispensable for imparting excellent thermal stability and reductionresistance to the oxide catalyst composition. For attaining thispurpose, it is necessary that the atomic ratio (b) of A, relative totwelve atoms of the total of molybdenum and tungsten, satisfy therelationship 0<b≦8.

B in formula (I) is at least one member selected from the groupconsisting of potassium (K), rubidium (Rb) and cesium (Cs). Element B isindispensable for not only further enhancing the effect of addition ofelement A, but also for improving the selectivity for methacrolein. Forattaining these purposes, it is necessary that the atomic ratio (c) ofB, relative to twelve atoms of the total of molybdenum and tungsten,satisfy the relationship 0<c<3. When the atomic ratio (c) of B becomes 3or more, it becomes impossible to obtain an oxide catalyst compositionhaving the desired catalytic activity even if not only the amount of atleast one element selected from the group consisting of potassium (K),rubidium (Rb) and cesium (Cs), but also the calcination and firingtemperatures are appropriately regulated. For obtaining an oxidecatalyst composition having the desired catalytic activity, it ispreferred that the atomic ratio (c) of B, relative to twelve atoms ofthe total of molybdenum and tungsten, is in the range of from more than0 to less than 2.0, more advantageously from more than 0 to less than1.5, still more advantageously from more than 0 to less than 1.2.

In the present invention, for obtaining remarkable improvements in thethermal stability and reduction resistance by the addition of element A(i.e., at least one member selected from the group consisting of La, Ce,Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y), while maintainingthe high selectivity for methacrolein, it is important to adjust therelationship between the amounts of element A, bismuth (Bi) and elementB (i.e., at least one element selected from the group consisting ofpotassium, rubidium and cesium). Specifically, it is important that theatomic ratios a, b and c in formula (I) satisfy the relationship0.02<b/(a+b+c)<0.6, preferably 0.05<b/(a+b+c)<0.5.

In addition, for synergistically improving the effect of adding elementB, it is important to adjust the relationship between the amounts ofelement B, bismuth and element A. Specifically, it is important that theatomic ratios a, b and c in formula (I) satisfy the relationship0<c/(a+b+c)≦0.9, preferably 0.1<c/(a+b+c)<0.8.

The reason why the excellent performances aimed at in the presentinvention can be achieved when amounts of bismuth, element A and elementB satisfy the above-mentioned requirements has not yet been elucidated.However, the reason is believed to be as follows. When bismuth, elementA and element B are used so as to satisfy a specific relationship,molybdic acid compounds respectively of bismuth, element A and element B(or such molybdic acid compounds as well as tungstenic acid compoundsrespectively of bismuth, element A and element B when tungsten is alsocontained in the oxide catalyst composition) undergo solidsolubilization, thereby exhibiting advantageous performances desired inthe present invention.

In the present invention, iron (Fe) is indispensable, similarly tobismuth, for commercial scale synthesis of methacrolein. However, whentoo large an amount of iron is contained in the oxide catalystcomposition, the amount of by-products, such as CO and CO₂, is likely toincrease, thus lowering the selectivity for methacrolein. Therefore, itis necessary that the atomic ratio (d) of iron, relative to twelve atomsof the total of molybdenum and tungsten, is in the range of from0.2<d<5.

Further, with respect to the iron component, it is important to adjustthe relationship between the amounts of iron, bismuth and element A, andit is necessary that the atomic ratios a, b and d in formula (I) satisfythe relationship 0.01<d/(a+b+d)≦0.9, preferably 0.2<d/(a+b+d)<0.9. Fromthe viewpoint of achieving high selectivity for methacrolein, it ispreferred that the atomic ratios a, b and d, relative to twelve atoms ofthe total of molybdenum and tungsten, satisfy the relationships 0.2<d<5and 0<d/(a+b+d)≦0.9, more preferably 0.2<d≦4 and 0.01<d/(a+b+d)≦0.9,still more preferably 0.2<d≦4 and 0.2<d/(a+b+d)<0.9.

In the oxide catalyst composition of the present invention which isrepresented by formula (I), X is cobalt solely, or a mixture of cobaltand at least one member selected from the group consisting of magnesiumand nickel. Cobalt (Co) X in formula (I) is indispensable for improvingthe catalytic activity of the oxide catalyst composition withoutlowering the selectivity for methacrolein. Specifically, it is necessarythat the atomic ratio (e) of X, relative to twelve atoms of the total ofmolybdenum and tungsten, satisfy the relationship 1≦e≦12.

In X of formula (I), magnesium (Mg) and nickel (Ni) are elements whichcan be used in partial substitution for the cobalt component. Withrespect to the cost of a starting material, a magnesium material and anickel material are less expensive than a cobalt material. Therefore, itis commercially advantageous that magnesium and/or nickel can be used inpartial substitution for the cobalt component, from the viewpoint ofreduction of catalyst production cost. However, if magnesium, nickel ora mixture of magnesium and nickel is used as X without being combinedwith cobalt, it is impossible to satisfactorily improve the catalyticactivity of the oxide catalyst composition. In the mixture X in formula(I), it is preferred that the atomic ratio of cobalt to the total ofcobalt, magnesium and nickel is 0.5 or more. When the mixture X informula (I) contains magnesium, the atomic ratio of magnesium to thetotal of cobalt, magnesium and nickel in the mixture X is preferably 0.5or less. When the mixture X in formula (I) contains nickel, the atomicratio of nickel to the total of cobalt, magnesium and nickel in themixture X is preferably less than 0.33.

In the present invention, antimony (Sb) is indispensable for suppressingthe selectivity for diacetyl and by-produced aldehydes, such asacetoaldehyde and acrolein. Specifically, it is necessary that theatomic ratio (f) of antimony, relative to twelve atoms of the total ofmolybdenum and tungsten, satisfy the relationship 0.1<f<3. For furtherimproving the catalytic activity of the oxide catalyst composition, itis preferred that the atomic ratio (f) of antimony satisfies therelationship 0.3≦f≦2.5.

For maintaining the selectivity for methacrolein, it is necessary thatthe amounts of antimony and iron satisfy a specific relationship.Specifically, it is necessary that d and f in formula (I) satisfy therelationship 0.1<d−f<2.5. For further improving the catalytic activityof the oxide catalyst composition, it is preferred that d and f informula (I) satisfy the relationship 0.3≦d−f≦2.3.

In addition, from the viewpoint of achieving a high selectivity formethacrolein, it is preferred that the atomic ratio (d) of iron and theatomic ratio (f) of antimony satisfy the relationships 0.3≦d−f≦2.3 and0.2<d/(a+b+d)<0.9 simultaneously.

Next, methods for producing the oxide catalyst composition of thepresent invention are explained in detail.

With respect to the methods for producing the oxide catalyst compositionof the present invention, there is no particular limitation, and anyconventional method can be used as long as it enables the production ofan oxide represented by formula (I) above. For example, the oxidecatalyst composition of the present invention can be produced by amethod which comprises the following steps of (1), (2) and (3).

In step (1), a slurry of starting materials as sources of elements usedin the oxide catalyst of the present invention is prepared. Examples ofsources of molybdenum, tungsten, bismuth, lanthanum, cerium,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, lutetium, yttrium,iron, cobalt, magnesium, nickel, potassium, rubidium and cesium includeammonium salts, nitrates, nitrites, chlorides, sulfates and organic acidsalts of these elements, which are soluble in water or nitric acid.Especially, it is preferred that a molybdenum source and a tungstensource are ammonium salts. It is also preferred that sources of bismuth,lanthanum, cerium, praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,lutetium, yttrium, iron, cobalt, magnesium, nickel, potassium, rubidiumand cesium are nitrates or nitrites of these elements. As sources ofantimony, there can be mentioned antimony pentaoxide, antimony trioxideand metallic antimony. From the viewpoint of availability, antimonytrioxide is preferred as an antimony source.

With respect to the methods for preparing a slurry of starting materialsas sources of elements, there is no particular limitation. For example,a slurry of starting materials can be prepared as follows. An aqueoussuspension of ammonium molybdate, ammonium tungstate and antimonytrioxide is prepared, and the prepared suspension is heated to 80° C. to90° C. while stirring, followed by the addition of hydrogen peroxide, tothereby obtain solution 1 containing molybdenum, tungsten and antimony.On the other hand, nitrates or nitrites of other elements are dissolvedin water or an aqueous solution of nitric acid to thereby obtainsolution 2. The obtained solution 2 is mixed with the above-mentionedsolution 1 containing molybdenum, tungsten and antimony, to therebyobtain a slurry of starting materials.

In step (2), the slurry obtained in the step (1) above is subjected tospray drying, to thereby obtain a spherical or quasisphericalparticulate catalyst precursor. The spray drying of the slurry can beconducted by a conventional method commercially employed, such ascentrifugation, a two-phase flow nozzle method or a high pressure nozzlemethod, to obtain a dried particulate catalyst precursor. In thisinstance, it is preferred to use air which has been heated by anelectric heater, steam or the like, as a heat source for drying. In thiscase, it is preferred that the entrance temperature of the dryer sectionof the spray dryer is from 150° C. to 400° C. By the use of the driedparticulate catalyst precursor thus obtained, it becomes possible toobtain the oxide catalyst composition in the form of an extrudedcatalyst, or preferably in the form of a tableted catalyst which ispreferred because of uniformity in shape and density thereof.

In step (3), the dried particulate catalyst precursor obtained in step(2) above is calcined and finally fired to thereby obtain a desiredoxide catalyst composition. The dried particulate catalyst precursor iscalcined at a temperature of from 180° C. to 400° C. for about 0.5 to 24hours, and, if desired, the resultant calcined product is molded into anappropriate shape by extrusion molding or tableting, followed by finalfiring at a temperature of from 350° C. to 600° C. for 1 to 24 hours.For calcination and final firing, a kiln, such as a rotary kiln, atunnel kiln or a muffle kiln, can be used.

From the viewpoint of improving the selectivity for the desired product,it is desired that no silica is used or, if used, the amount of silicain the oxide catalyst composition is as small as possible. However, whenit is desired to increase the surface area of the oxide catalystcomposition so as to improve the activity thereof, silica may be used ina limited amount. Examples of silica sources include silica sol, silicagel, and a silicate, such as potassium silicate or sodium silicate. Inthe oxide catalyst composition, it is preferred that the atomic ratio ofsilica, relative to twelve atoms of the total of molybdenum andtungsten, is 3 or less, more preferably 1 or less, still more preferably0.1 or less, in terms of silicon (Si).

The oxide catalyst composition of the present invention is a catalystfor use in producing methacrolein or a mixture of methacrolein andmethacrylic acid by reacting at least one member selected from the groupconsisting of isobutylene and t-butanol with a molecularoxygen-containing gas. With respect to the methods for producingmethacrolein or a mixture of methacrolein and methacrylic acid by usingthe oxide catalyst composition of the present invention, there is noparticular limitation. However, hereinbelow, an explanation is made withrespect to a preferred method of using the oxide catalyst composition ofthe present invention.

The gaseous phase catalytic oxidation reaction of at least one memberselected from the group consisting of isobutylene and t-butanol with amolecular oxygen-containing gas can be carried out by introducing afeedstock gas (comprising 1% to 10% by volume of isobutylene, t-butanolor a mixture thereof and 99% to 90% by volume of a gaseous mixture of amolecular oxygen-containing gas and a diluent gas) to a fixed bedreactor having a fixed catalyst bed of a (preferably tableted) catalystcomprised of the above-mentioned oxide catalyst composition, wherein thefeedstock gas is introduced at a temperature of from 250° C. to 450° C.under a pressure of from atmospheric pressure to 5 atm and at a spacevelocity of from 400 to 4,000/hr (under normal temperature and pressure(NTP) conditions).

Examples of molecular oxygen-containing gases include pure oxygen gas,and an oxygen-containing gas, such as air. Examples of diluent gasesinclude nitrogen, carbon dioxide, steam and a mixture thereof.

In the present invention, it is preferred that the volume ratio of themolecular oxygen-containing gas to the above-mentioned gaseous mixtureof the molecular oxygen-containing gas and the diluent gas satisfy therequirements of the formula 0.04<molecular oxygen-containinggas/(molecular oxygen-containing gas+diluent gas)<0.3, and that theconcentration of molecular oxygen in the feedstock gas is from 4% to 20%by volume.

For preventing the occurrence of coking on the catalyst composition, itis necessary that steam be contained in the feedstock gas. However, fromthe viewpoint of suppressing the by-production of carboxylic acids, suchas methacrylic acid, acetic acid and acrylic acid, it is preferred thatthe concentration of steam in the diluent gas is reduced to a level aslow as possible. It is preferred that the amount of steam in thefeedstock gas is generally from more than 0% to 30% by volume.

The oxide catalyst composition of the present invention exhibits notonly a prolonged catalyst life due to its excellent properties withrespect to thermal stability and reduction resistance, but alsoexcellent selectivity for the desired product. By the use of the oxidecatalyst composition of the present invention for producing methacroleinor a mixture of methacrolein and methacrylic acid, it becomes possibleto stably produce the desired product for a long time while holding downthe amount of the by-produced impurities, e.g. diacetyl. Methacroleinproduced using a conventional catalyst contains a large amount (severalthousands ppm) of diacetyl. By contrast, it has been found that thediacetyl content of methacrolein produced by the use of the oxidecatalyst composition of the present invention is as low as 900 ppm orless.

In addition, the present inventors have analyzed each of methacroleinand a mixture of methacrolein and methacrylic acid by gaschromatography. The results of the analysis are as follows. A peakascribed to diacetyl is detected at a retention time of about 17.3minutes, and also two unidentified peaks are, respectively, detected atretention times of about 22.0 minutes and about 39.2 minutes (theseunidentified peaks are, respectively, designated “R1 ” and “R2”). Thearea of each of the peaks R1 and R2 has been compared to the peak areaof diacetyl, and the percentages of the areas of R1 and R2, each basedon the peak area of diacetyl, are, respectively, designated “S1” and“S2”. From S1 and S2 values it has been found that both of R1 and R2 areascribed to substances causative of discoloration. Specifically, it hasbeen confirmed, by the studies of the present inventors, that, even inthe case of methyl methacrylate containing by-produced diacetyl in anamount as low as 650 ppm, a discoloration tends to occur when S1 valueis 50% or more and S2 value is 80% or more, each based on the peak areaof diacetyl. The oxide catalyst composition of the present invention iscapable of holding down not only the formation of diacetyl, but also theformation of the unidentified impurities to which R1 and R2 areascribed.

The above-mentioned methacrolein or mixture of methacrolein andmethacrylic acid, which has a very low content of impurities, is veryuseful as a raw material for producing methyl methacrylate havingexcellent transparency.

As the methods actually practiced for the commercial-scale production ofmethyl methacrylate, there can be mentioned a “via methacrylic acidprocess” comprising three reaction steps and a “direct ML-to-MMAprocess” comprising two reaction steps.

The via methacrylic acid process is described at pages 172 to 176 of“Sekiyu Kagaku Prosesu (Petrochemical Process)”, published by KodanshaScientific, Inc., Japan. The above-mentioned document states that thevia methacrylic acid process comprises three reaction steps, that is, afirst oxidation step, a second oxidation step and an esterificationstep. The first oxidation step is a step of subjecting at least onestarting material selected from the group consisting of isobutylene andt-butanol to a gaseous phase catalytic oxidation reaction with molecularoxygen in the presence of a catalyst, to thereby obtain methacrolein.The second oxidation step is a step of subjecting the methacroleinobtained in the first oxidation step to a gaseous phase catalyticoxidation reaction with molecular oxygen in the presence of a catalyst,to thereby obtain methacrylic acid. The esterification step is a step ofsubjecting the methacrylic acid obtained in the second oxidation step toesterification, to thereby obtain methyl methacrylate.

The oxide catalyst composition of the present invention can be appliedto the via methacrylic acid process in a manner in which the firstoxidation step is performed using the oxide catalyst composition of thepresent invention to thereby produce methacrolein, and subsequently thesecond oxidation step and the esterification step are performed in aconventional manner, thereby obtaining methyl methacrylate. When thefirst oxidation step is performed using the oxide catalyst compositionof the present invention, methacrolein having only low contents ofimpurities can be produced. Therefore, the adverse influences of suchimpurities on the methacrolein oxidation catalyst used in the secondoxidation step (such as lowering of the catalytic activity andshortening of the catalyst life) can be decreased. In addition, thefollowing should be noted. Since the reaction temperature used in thesecond oxidation step of the via methacrylic acid process is high, thatis, 300° C. to 400° C., it is considered that most of the substancescausative of discoloration which are contained in the methacrolein aredecomposed during the second oxidation step of the via methacrylic acidprocess. However, since not all of the substances causative ofdiscoloration are decomposed in the via methacrylic acid process, use ofmethacrolein containing no substances causative of discoloration isdesired for producing excellent methyl methacrylate.

The direct ML-to-MMA process comprises two catalytic reaction steps,wherein the first reaction step comprises subjecting isobutylene and/ort-butanol as a starting material to a gaseous phase catalytic oxidationreaction with a molecular oxygen-containing gas in the presence of anoxide catalyst (hereinafter, this catalyst is frequently referred to asa “first stage catalyst”) to thereby obtain methacrolein, and the secondreaction step comprises subjecting the obtained methacrolein to agaseous phase catalytic reaction with methanol and a molecularoxygen-containing gas in the presence of a carrier-supported catalystcontaining palladium (hereinafter, this catalyst is frequently referredto as a “second stage catalyst”), to thereby produce methyl methacrylate(MMA) by one step directly from methacrolein (ML). The oxide catalystcomposition of the present invention is useful as a first stage catalystin the direct ML-to-MMA process. Specifically, production ofmethacrolein or a mixture of methacrolein and methacrylic acid, that is,the first reaction step, can be performed by the above-mentionedpreferred method.

In the second reaction step of the direct ML-to-MMA process, themethacrolein obtained in the first reaction step is reacted withmethanol, wherein the methacrolein used contains only small amounts ofimpurities, namely methacrolein produced using the oxide catalystcomposition of the present invention, to thereby produce methylmethacrylate. As the second stage catalyst used in the second reactionstep, there can be used palladium-containing catalysts disclosed in, forexample, U.S. Pat. No. 4,249,019, Examined Japanese Patent ApplicationPublication No. Sho 57-35859, U.S. Pat. No. 4,518,796 and InternationalPublication No. WO 97/3751. In addition, the second reaction step can beconducted in accordance with the reaction modes disclosed in thesepatent documents. Specifically, methyl methacrylate can be produced byreacting molecular oxygen, methacrolein and methanol in the presence ofa second stage catalyst under moderate reaction conditions wherein thereaction temperature is in the range of from room temperature to 100° C.Thus, the second reaction step of the direct ML-to-MMA process isconducted at a low temperature at which most of the impurities, e.g.diacetyl, do not undergo a reaction. As explained above in detail, whena conventional first stage catalyst is used in the first reaction stepof the direct ML-to-MMA process, the impurities, e.g. diacetyl, areby-produced in a large amount. Production of methyl methacrylatecontaining only small amounts of the impurities causative ofdiscoloration has for the first time become possible by using the oxidecatalyst composition of the present invention as a first stage catalystin the first reaction step. When the oxide catalyst composition of thepresent invention is used in the first reaction step, the methylmethacrylate obtained in the second reaction step has an extremely lowimpurity content and, thus, there is no need to repeatedly conduct thesubsequent purification operation. As a result, the loss of methylmethacrylate becomes decreased, leading to a cost reduction.Accordingly, the oxide catalyst composition of the present invention isvery advantageous for the commercial production of methyl methacrylate.

The first stage and second stage reactions of the direct ML-to-MMAprocess and the operation for purifying methyl methacrylate may beconducted in a continuous manner. As an example of a continuous reactionmode, there can be mentioned the reaction mode involved in the directML-to-MMA process conducted in the Examples of the present applicationfor producing methyl methacrylate, that is, the process shown in theflow chart of the Figure.

Methyl methacrylate having excellent transparency can be produced by thedirect ML-to-MMA process using the oxide catalyst composition of thepresent invention as a first stage catalyst. As apparent from theresults of the Examples and Comparative Examples of the presentapplication, methyl methacrylate monomer produced using the oxidecatalyst composition of the present invention has an APHA value(determined in accordance with JIS-K6716) of not more than 5, and methylmethacrylate polymer obtained by polymerizing such methyl methacrylatemonomer has a YI value (determined in accordance with JIS-K7103) of notmore than 10.

Such methyl methacrylate polymer having excellent transparency can beused as a substitute for glass and quartz in application fieldsrequiring high transparency, such as optical fibers, light guide platesand the like; thus, such highly transparent methyl methacrylate polymerhas very high commercial value.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be described in more detail withreference to the following Examples and Comparative Examples, but theyshould not be construed as limiting the scope of the present invention.

The number of oxygen atoms in the oxide catalyst composition isdetermined depending on the valence requirements of the other elementspresent. Therefore, in Examples and Comparative Examples, the indicationof oxygen atom in the oxide catalyst composition is omitted from theformula representing the oxide catalyst composition.

In the following Examples and Comparative Examples, various propertiesof the oxide catalyst composition were evaluated as follows.

<Conversion and Selectivity>

The conversion and selectivity used for evaluating the results of thereaction are defined as follows:${{Conversion}{\mspace{11mu}\;}(\%)} = {\frac{\begin{matrix}{{mole}\mspace{14mu}{of}\mspace{14mu}{isobutylene}\mspace{14mu}{or}} \\{t\text{-}{butanol}\mspace{14mu}{reacted}}\end{matrix}}{\begin{matrix}{{mole}\mspace{14mu}{of}\mspace{14mu}{isobutylene}\mspace{14mu}{or}} \\{t\text{-}{butanol}\mspace{14mu}{charged}}\end{matrix}} \times 100}$${{Selectivity}\mspace{14mu}(\%)} = {\frac{\begin{matrix}{{mole}\mspace{14mu}{of}\mspace{14mu}{methacrolein}\mspace{14mu}{or}} \\{{{methacrylic}\mspace{14mu}{acid}\mspace{14mu}{formed}}\;}\end{matrix}}{\begin{matrix}{{mole}\mspace{14mu}{of}\mspace{14mu}{isobutylene}\mspace{14mu}{or}} \\{t\text{-}{butanol}\mspace{14mu}{reacted}}\end{matrix}} \times 100}$Data used to calculate the conversion and selectivity were obtained bygas chromatography.<Determination of the Impurities Causative of Discoloration>

The amounts of the impurities causative of discoloration, which arecontained in methacrolein or a mixture of methacrolein and methacrylicacid, were determined by gas chromatography as follows. As an apparatusfor gas chromatography, there was used gas chromatograph GC-17A(manufactured and sold by Shimadzu Corporation, Japan) which wasequipped with a capillary column of 110 m in length. The capillarycolumn was obtained by connecting in series the following threecapillary columns each having an inner diameter of 0.25 mm: TC-1(length: 60 m), DB-1 (length: 30 m) and TG-WAX (length: 20 m). The gaschromatography was conducted under conditions wherein the initialtemperature of the column was 45° C., and, after maintaining the columnat 45° C. for 30 minutes, the column temperature was elevated to 220° C.at a temperature elevation rate of 5° C./min, and maintained at 220° C.for 25 minutes. A sample for gas chromatography was prepared bycondensing all of the gaseous product containing methacrolein or amixture of methacrolein and methacrylic acid, to thereby obtain acondensate, and adding 1,2-dimethoxyethane as an internal standard tothe obtained condensate.

When the gas chromatography was performed under the above-mentionedconditions, a peak ascribed to diacetyl was detected at a retention timeof about 17.3 minutes. In addition, two unidentified peaks were,respectively, detected at retention times of about 22.0 minutes andabout 39.2 minutes, and these unidentified peaks were, respectively,named “R1” and “R2”. The area of each of the peaks R1 and R2 is comparedto the peak area of diacetyl, and the percentages of the areas of R1 andR2, each based on the peak area of diacetyl, are, respectively,designated “S1” and “S2”. The S1 and S2 values are, respectively, usedas indices for the by-production of R1 and R2. (Hereinafter, forsimplicity's sake, the unit “%” is omitted from the S1 and S2 values.)

For producing methyl methacrylate having excellent transparency by thedirect ML-to-MMA process, it is preferred that the amount of diacetylcontained in methacrolein is not more than 900 ppm, more advantageouslynot more than 600 ppm.

<Evaluation of the Discoloration of a Methyl Methacrylate Monomer>

The level of discoloration of a methyl methacrylate monomer wasevaluated in accordance with JIS-K6716. Several dilutions of aconcentrated hydrochloric acid solution containing potassiumhexachloroplatinate (IV) and cobalt chloride were prepared to obtain aset of standard solutions. The APHA value of a sample solution wasdetermined by using the set of the standard solutions as a criterion,wherein the standard solutions had specific different APHA valuesrespectively corresponding to the degrees of dilution of thehydrochloric acid solution. Specifically, distilled water was used as astandard solution for an APHA value of 0, and standard solutions forAPHA values of 5, 10, 15 and 20 were prepared by using correspondingdifferent dilutions of the hydrochloric acid solution, wherein thesmaller the degree of dilution, the higher the APHA value. The APHAvalue of a methyl methacrylate monomer was evaluated by comparing thecolor of the methyl methacrylate monomer with those of the standardsolutions, and the APHA value was used as an index for discoloration.

The degree of discoloration of a methyl methacrylate monomer ispreferably 5 or less, in terms of the APHA value. A methyl methacrylatepolymer having excellent transparency can be obtained by polymerizingsuch a methyl methacrylate monomer having an APHA value of 5 or less.

<Evaluation of the Discoloration of a Methyl Methacrylate Polymer>

The level of discoloration of a methyl methacrylate polymer, which isobtained by polymerizing a methyl methacrylate monomer, was evaluated inaccordance with the tests described in JIS-K7103, namely the tests fordetermining the yellowness index and yellowing factor of plastics.

A test specimen which was a methyl methacrylate polymer plate having alength of 55 cm, a width of 10 cm, and a thickness of 5 mm was preparedas follows. A gasket was sandwiched between two glass plates, and theglass plates were clamped together with the gasket held therebetween,thereby forming a uniform thickness space between the two glass plates,wherein the space functions as a cavity. A methyl methacrylate monomercontaining added thereto 0.05% by weight of 2,2′-azo-bisisobutyronitrileas a polymerization initiator was poured into the space between the twoglass plates by using a funnel. The glass plates were clamped further soas to remove air remaining between the glass plates and, then, themethyl methacrylate sandwiched between the glass plates was hermeticallysealed. The resultant structure (comprising a methyl methacrylate layersandwiched between the glass plates) was placed in a warm water bath at50±1° C. for 6 hours and, then, placed in a thermostat, constanttemperature bath maintained at 115±1° C. for 2 hours, to effect apolymerization of the methyl methacrylate, thereby obtaining a methylmethacrylate polymer. The thus obtained methyl methacrylate polymer(polymer plate) was allowed to cool to room temperature, and the polymerplate (thickness: 5 mm) was released from the glass plates. The polymerplate was cut into a size of 55 cm in length and 10 cm in width. Then,the two opposite 10 cm-length edge surfaces of the polymer plate werepolished using a file and a buff, to thereby obtain a test specimen.

The obtained test specimen was visually examined with respect to the 10cm-length edge surfaces thereof. Further, the test specimen was analyzedby a long optical path type spectrotransmission colorimeter (ASA-2Model, manufactured and sold by Nippon Denshoku Industries, Co., Ltd.,Japan). From the data obtained using the long optical path typespectrotransmission colorimeter, the yellowness index (YI) wascalculated in accordance with JIS-K7103 (describing the tests fordetermining the yellowness index and yellowing factor of plastics). TheYI value was used as an index for the discoloration of a methylmethacrylate polymer.

A methyl methacrylate polymer having a yellowness index (YI) of not morethan 10 is preferred because such a polymer has excellent transparency.

EXAMPLE 1

An oxide catalyst composition having a structure (in terms of atomicratios of constituent metallic elements, relative to twelve atoms of thetotal of molybdenum and tungsten) represented by the formula:Mo₁₂Bi_(1.6)Ce_(0.4) K_(0.1)Cs_(0.4)Fe_(1.5)CO_(8.0)Sb_(0.7)was prepared as follows.

Ammonium heptamolybdate in an amount of 350.0 g was dissolved in 1,700 gof water having a temperature of about 50° C., to thereby obtain anaqueous solution. To the obtained aqueous solution was added 16.8 g ofantimony trioxide, thereby obtaining an aqueous suspension containingmolybdenum and antimony. The obtained aqueous suspension was heated to90° C. while stirring, and then 65.0 g of a 30% by weight aqueoushydrogen peroxide solution was slowly added thereto while stirring. Theresultant mixture turned to assume bright yellow while foaming, and thestate of the mixture became a solution. The stirring of the thusobtained aqueous solution was further continued for about 30 minutes at90° C. Then, the solution was cooled to 50° C. and the solution wasmaintained at 50° C. (the thus obtained solution is referred to as“solution A”). On the other hand, 128.7 g of bismuth nitrate, 29.1 g ofcerium nitrate, 1.66 g of potassium nitrate, 100.4 g of iron nitrate,389.0 g of cobalt nitrate and 12.9 g of cesium nitrate were dissolved in350 g of a 15% by weight aqueous nitric acid solution, to thereby obtaina solution (referred to as “solution B”). Solutions A and B were mixedtogether while stirring for about 2 hours, thereby obtaining a slurry ofthe starting materials. The obtained slurry was subjected to spraydrying, to thereby obtain a dried, particulate catalyst compositionprecursor. The obtained dried, particulate catalyst compositionprecursor was calcined at 200° C. for 3 hours, to thereby obtain acalcined catalyst composition precursor in the form of a quasisphericalparticle. The obtained calcined catalyst composition precursor wasmolded into a columnar tablet having a diameter of 5.0 mm and a heightof 4 mm, and the tablet was subjected to final firing at 520° C. for 3hours, thereby obtaining a final tableted oxide catalyst composition.

Methacrolein was produced in order to evaluate the initial performancesof the oxide catalyst composition. 4.0 g of the tableted oxide catalystcomposition was charged into a stainless steel (SUS304) reaction tubewhich has a diameter of 10 mm and is provided with a jacket. A gaseousmixture of 6% by volume of isobutylene, 10.8% by volume of oxygen, 10.0%by volume of steam and 73.2% by volume of nitrogen was flowed to thereactor at a flow rate of 100 ml/min (NTP) while maintaining theinternal temperature of the reactor at 350° C., thereby effecting amethacrolein synthesizing reaction and obtaining a gaseous productcontaining methacrolein. The results of the reaction were evaluated, andit was found that the conversion of isobutylene was 97.8%, theselectivity for methacrolein was 88.3% and the selectivity formethacrylic acid was 2.4%. The analysis of the condensed gaseous productshowed that the production of diacetyl was 500 ppm, the S1 value was 10,and the S2 value was 52.

Subsequently, a test under stringent conditions was performed asfollows. The reaction temperature was elevated to 480° C. and the flowrate of the above-mentioned gaseous mixture was changed to 220 ml/min(NTP), and a continuous operation for synthesizing methacrolein wasconducted for 48 hours. Subsequently, the reaction conditions werechanged back to the same conditions as in the reaction for theevaluation of the initial performances of the catalyst composition(reaction temperature: 350° C.; flow rate of gaseous mixture: 100ml/min), and the results of the reaction are shown below. It was foundthat the performances of the catalyst composition under the stringentconditions were substantially the same as the initial performances.Specifically, the conversion of isobutylene was 97.8%, the selectivityfor methacrolein was 88.3% and the selectivity for methacrylic acid was2.4%. The analysis of the condensed gaseous product showed that theproduction of diacetyl was 490 ppm, the S1 value was 10, and the S2value was 51.

In addition, using the above-mentioned oxide catalyst composition,methyl methacrylate was produced by the direct ML-to-MMA process inaccordance with the steps as shown in the flow chart of Figure.

1. First Stage Reaction Step:

The first stage reaction was conducted in a manner as shown below,making reference to the reaction method of Example 1 of UnexaminedJapanese Patent Application Laid-Open Specification No. Hei 9-323950.

The same tableted catalyst composition as prepared above was chargedinto a stainless steel (SUS304) reaction tube which has an outerdiameter of 50.7 mm and an inner diameter of 46.7 mm and is providedwith a jacket. Specifically, the reaction tube was packed with thetableted catalyst composition so as to form three catalyst layerstherein, namely catalyst layers 1, 2 and 3 disposed in this order fromthe gas inlet to the gas outlet of the reaction tube, thereby providingreaction zones 1, 2 and 3 corresponding to catalyst layers 1, 2 and 3,respectively. The packing of the catalyst composition was performed sothat the catalyst packing densities (C1, C2 and C3 respectively) of thecatalyst layers 1, 2 and 3 became as follows: C1=800 kg/m³, C2=400 kg/m³and C3=1,000 kg/m³, and the heights (L1, L2 and L3 respectively) of thecatalyst layers 1, 2 and 3 became as follows: L1=0.6 m, L2=1.5 m andL3=2.5 m. The adjustment of the catalyst packing densities of thecatalyst layers 1, 2 and 3 was made by mixing a cylindrical porcelainRaschig ring (diameter: 5 mm, height: 4 mm, through-hole diameter: 3 mm)with the tableted catalyst.

The temperature of the heating medium of the jacket was set at 320° C.,and a gaseous mixture of 5.75% by volume of t-butanol (TBA), 8.37% byvolume of oxygen, 4.17% by volume of steam and 81.7% by volume ofnitrogen (wherein the temperature of the gaseous mixture was 290° C.)was flowed to the reaction tube, and a methacrolein synthesizingreaction was conducted at a space velocity (SV) of 630 hr⁻¹, therebypreparing a gas containing methacrolein and steam. The maximumtemperatures of the reaction zones 1, 2 and 3 were 383° C., 384° C. and384° C., respectively. The conversion of t-butanol was 100%, theselectivity for methacrolein was 86.4% and the production of diacetylwas 500 ppm.

2. Methacrolein (MAL) Absorption Step:

Next, a methacrolein absorption step was conducted in a manner as shownbelow, wherein a quenching tower, a dehydration tower and an absorptiontower were used, making reference to the disclosure of UnexaminedJapanese Patent Application Laid-Open Specification No. Hei 11-80077(corresponding to U.S. Pat. No. 5,969,178).

The methacrolein- and steam-containing gas prepared in step 1 above wasintroduced into a quenching tower. In the quenching tower, the gas wascooled to a temperature of 44° C. by means of water, so that a largeportion of the steam and by-produced high boiling point substances andacids was removed from the gas, to thereby partially dehydrate the gas.The resultant partially dehydrated, methacrolein-containing gas had thefollowing composition: 4.9 mol % of methacrolein, 2.7 mol % of water,0.2 mol % of liquid by-products, such as acetone, and 92.2 mol % intotal of nitrogen, oxygen, carbon dioxide and carbon monoxide gases andunreacted isobutylene.

The partially dehydrated, methacrolein-containing gas was fed into thebottom portion of a 30-stage plate dehydration tower (inner diameter: 10cm, height: 5 m) provided with sieve trays at a flow rate of 3.6 Nm³/hr,whereas a solution prepared by adding 100 ppm by weight of hydroquinoneto liquid methanol was fed into the dehydration tower at the top platethereof at a flow rate of 200 g/hr. The dehydration tower was operatedunder conditions wherein the temperature of the gas in the bottomportion of the dehydration tower was 44° C., the temperature of the gasin the uppermost portion of the dehydration tower was 18° C., thetemperature of the liquid methanol solution was 18° C., and the pressurein the uppermost portion of the dehydration tower was 1.5 kg/cm². Thus,the partially dehydrated gas was dehydrated further under theabove-mentioned conditions, and a dehydrated gaseous mixture containingmethacrolein gas and methanol gas was obtained from the uppermostportion of the dehydration tower.

Subsequently, the dehydrated gaseous mixture obtained above was fed intothe bottom gas phase portion of a 30-stage plate absorption tower (innerdiameter: 10 cm, height: 5 m) provided with sieve trays, whereas asolution prepared by adding 100 ppm by weight of hydroquinone to liquidmethanol was fed into the absorption tower at the top plate thereof at aflow rate of 900 g/hr. The absorption tower was operated underconditions wherein the temperature of the liquid in the bottom portionof the absorption tower was −6° C., the temperature of the liquid on thetop plate was −3° C., the temperature of the liquid methanol solutionwas −3° C., and the pressure in the uppermost portion of the absorptiontower was 1.4 kg/cm². Substantially all of the methacrolein gas andmethanol gas which were contained in the dehydrated gaseous mixture wasabsorbed by liquid compounds under the above-mentioned conditions,thereby obtaining a liquid mixture (A) containing liquid methacroleinand liquid methanol from the bottom portion of the absorption tower. Theobtained liquid mixture had the following composition: 31.7% by weightof methacrolein, 66.8% by weight of methanol, 0.7% by weight of water,and 0.8% by weight of by-products, such as acetone.

3. Second Stage Reaction Step:

The second stage catalyst was prepared in accordance with ReferenceExample 1 and Example 1 of International Patent Application PublicationNo. WO97/3751, as follows.

A catalyst intermediate was prepared in substantially the same manner asin Reference Example 1 of International Patent Application PublicationNo. WO97/3751. Aluminum nitrate and magnesium nitrate were dissolved inan aqueous silica sol (Snowtex N-30 (SiO₂ content: 30% by weight),manufactured and sold by Nissan Chemical Industries, Ltd., Japan) sothat the Al/(Si+Al) proportion became 10 mol % and the Mg/(Si+Mg)proportion became 10 mol %. The resultant solution was subjected tospray drying at 130° C. using a spray dryer, thereby obtaining sphericalparticles having an average particle diameter of 60 μm. The obtainedparticles were calcined in air at 300° C. for 2 hours and subsequentlyat 600° C. for 3 hours, thereby obtaining a carrier for a catalyst. Tothe obtained carrier were added an aqueous solution of palladiumchloride (15% by weight) and sodium chloride (10 t by weight) and anaqueous solution of lead nitrate (13% by weight) so that the amounts ofthe palladium chloride and lead nitrate contained in the resultantmixture became 5 parts by weight in terms of Pd atom and 6.5 parts byweight in terms of Pb atom, respectively. The resultant mixture wasstirred at room temperature for 1 hour, thereby obtaining a carrierhaving adsorbed thereon almost all amounts of the palladium chloride andlead nitrate. Thereafter, to the obtained carrier having adsorbedthereon the palladium chloride and lead nitrate was dropwise added,while stirring, an aqueous solution containing hydrazine in a molaramount which is 3 times the total molar amount of the palladium chlorideand lead nitrate adsorbed on the carrier, thereby reducing the palladiumchloride and lead nitrate adsorbed on the carrier. Thus, a compositionPd^(5.0)Pb^(6.5)/SiO₂—Al₂O₃—MgO, wherein the superscript numerals at theright hand of Pd and Pb, respectively, represent parts by weight of Pdand Pb, relative to 100 parts by weight of the carrier, was obtained(hereinafter, the obtained composition is frequently referred to simplyas “catalyst intermediate”). The analysis of the catalyst intermediaterevealed that the atomic ratio of palladium to lead (Pd/Pb atomic ratio)was 3/1.95, and a maximum intensity peak at a diffraction angle (2θ) was38.745° in the powder X-ray diffraction pattern thereof, and the ratioof the total intensity of two peaks ascribed to metallic palladium 3delectrons to the intensity of a peak ascribed to metallic lead 4felectrons in the X-ray photoelectron spectrum was 1/1.24.

Next, the catalyst intermediate was activated by the method as describedin Example 1 of International Patent Application Publication No.WO97/3751, as follows. An external circulation type bubble columnreactor (made of stainless steel) which was equipped with a separatorfor a catalyst and had a volume of 5.0 liters for a liquid phase, wascharged with 1,200 g of the catalyst intermediate. A 36.7% by weightsolution of methacrolein in methanol and a 2 to 4% by weight solution ofNaOH in methanol were continuously fed to the reactor at flow rates of2.16 liters/hr and 0.24 liter/hr, respectively (the methacroleinconcentration of the reaction system comprised of the above-mentionedtwo different solutions was about 33% by weight), while introducing airinto the reactor so that the oxygen concentration at the outlet of thereactor became 3.0% by volume (which is equivalent to the oxygen partialpressure of 0.15 kg/cm²), to effect an activation of the catalystintermediate. The activation was conducted at a temperature of 80° C.under a pressure of 5 kg/cm². The concentration of NaOH in theabove-mentioned methanol solution was controlled so that the reactionsystem had a pH of 7.1. The reaction mixture (activated catalyst) wascontinuously withdrawn from the outlet of the reactor at a rate of 2.4liters/hr. The activation of the catalyst intermediate was completedafter 50 hours from the start of the reaction. During the reaction, thereaction mixture fractions continuously withdrawn from the outlet of thereactor contained about 270 ppm of lead on the average. It is assumedthat the reasons for this is that the lead contained in the catalyst isdissolved out in an ionized form thereof into the reaction system by theaction of the methacrylic acid produced during the above reaction (thereaction mixture fractions continuously withdrawn from the outlet of thereactor contained 1.1% by weight of methacrylic acid on the average),and the formed lead ions are reduced with active hydrogen which isgenerated in the reaction between methacrolein and methanol. Theanalysis of the activated catalyst revealed that the Pd/Pb atomic ratiowas 3/1.24, and a maximum intensity peak at a diffraction angle (2θ) was38.652° in the powder X-ray diffraction pattern thereof.

The second stage reaction was conducted using the liquid mixture (A)obtained in step 2 above (methacrolein absorption step) and theabove-prepared second stage catalyst, making reference to the Examplesof Unexamined Japanese Patent Application Laid-Open Specification No.Hei 11-80077 (corresponding to U.S. Pat. No. 5,969,178).

Two external circulation type bubble column reactors (reactor I andreactor II which were made of stainless steel) each having a volume of5.0 liters for a liquid phase were connected in series (i.e., reactor IIwas connected to reactor I). Each of reactors I and II was individuallycharged with 900 g of the activated second stage catalyst. The liquidmixture (A) obtained in step 2 above was fed into reactor I at a flowrate of 1,600 g/hr. In this instance, a solution of sodium hydroxide inmethanol and a solution of lead acetate in methanol were also fed intoeach of reactors I and II so that the liquid in each reactor had a pHvalue of 6.1 and a lead concentration of 20% by weight. An oxidativeesterification reaction of methacrolein was conducted at a temperatureof 80° C. under a pressure of 3.0 kg/cm², wherein the oxygen partialpressures of the exhausted gases flowing out from the outlets ofreactors I and II were 0.095 kg/cm² and 0.03 kg/cm², respectively. As aresult, a reaction mixture (B) containing methyl methacrylate, water,methacrolein and methanol was obtained from the outlet of reactor II,and it was found that the conversion of methacrolein was 80.3% and theselectivity for methyl methacrylate was 90.7%.

4. Methacrolein (MAL) Recovery Step

Methacrolein recovery step was conducted as follows, making reference toExample 1 of Unexamined Japanese Patent Application Laid-OpenSpecification No. Hei 11-246453. The reaction mixture (B) containingmethyl methacrylate, water, methacrolein and methanol, which wasobtained in step 3 (second stage reaction step) above, was fed into a45-stage plate distillation tower (inner diameter: 15 cm, height: 6 m)provided with sieve trays. The reaction mixture (B) was fed at the 30thplate as counted from the top of the distillation tower at a flow rateof 1,600 g/hr. Hydroquinone as a polymerization inhibitor was fed intothe distillation tower at the top thereof at a rate such that theconcentration of the polymerization inhibitor in the liquid fallinginside the distillation tower became at least 100 ppm. The distillationtower was operated under conditions wherein the temperature at theuppermost portion of the distillation tower was 31° C., the temperatureat the lowermost portion of the distillation tower was 84° C., thetemperature at the 6th plate as counted from the bottom of thedistillation tower was 81.4° C., and the pressure at the uppermostportion of the distillation tower was atmospheric pressure. As a result,a bottom liquid (C) containing methyl methacrylate was obtained from thebottom of the distillation tower.

5. Acid Treatment/Water-Oil Separation Step

The bottom liquid (C) obtained in step 4 above was fed into an oil-waterphase separation vessel at a flow rate of 800 g/hr. Aqueous sulfuricacid was fed into the conduit for feeding bottom liquid (C) to theoil-water phase separation vessel so that the resultant aqueous phase inthe oil-water phase separation vessel had a pH of 2.0. The mixture ofbottom liquid (C) and sulfuric acid in the separation vessel wasseparated into an oil phase and an aqueous phase by centrifugation, andthe oil phase was recovered and subjected to the subsequent high boilingpoint substance separation step. In this step 5, two separation vesselswere used which were disposed so as to enable alternate use thereof forconducting the phase separation of the bottom liquid (C).

6. High Boiling Point Substance Separation Step

A high boiling point substance separation step was conducted as follows,making reference to Example 1 of Unexamined Japanese Patent ApplicationLaid-Open Specification No. Hei 11-302224. The oil phase separated instep 5 above was fed into a 30-stage plate distillation tower (innerdiameter: 10 cm, height: 5 m) provided with sieve trays at the 20thplate as counted from the top thereof at a flow rate of 600 g/hr. Anapparatus comprising a brine refrigerator for removing a liquid underreduced pressure was provided at the top of the distillation tower, andan apparatus which was controlled by means of a level gauge and whichcomprised a refrigerator for withdrawing steam under reduced pressureand cooling the withdrawn steam to thereby obtain a condensate, wasprovided at the bottom of the distillation tower. The distillation towerwas continuously operated under conditions wherein the reflux rate was1,000 g/hr and the pressure at the uppermost portion of the distillationtower was 150 mmHg, while feeding a methyl methacrylate solutioncontaining 5% by weight of hydroquinone from the top of the distillationtower at a flow rate of 40 g/hr. The temperatures at the uppermostportion and the lowermost portion of the distillation tower were 45° C.and 70° C., respectively. A steam containing methyl methacrylate wasrecovered from the top of the distillation tower and cooled by means ofa refrigerator to thereby obtain a condensate. The condensate waswithdrawn at a rate of 500 g/hr.

7. Low Boiling Point Substance Separation Step

A low boiling point substance separation step was conducted as follows,making reference to Example 1 of Unexamined Japanese Patent ApplicationLaid-Open Specification No. Hei 11-35523. The condensate obtained instep 6 above was fed into a 30-stage plate distillation tower (innerdiameter: 10 cm, height: 5 m) provided with sieve trays at the 10thplate as counted from the top thereof at a flow rate of 500 g/hr. Anapparatus comprising a brine refrigerator for removing a liquid underreduced pressure was provided at the top of the distillation tower, andan apparatus which was controlled by means of a level gauge and whichcomprised a refrigerator for withdrawing steam under reduced pressureand cooling the withdrawn steam to thereby obtain a condensate, wasprovided at the bottom of the distillation tower. The distillation towerwas continuously operated under conditions wherein the reflux rate was400 g/hr and the pressure at the uppermost portion of the distillationtower was 250 mmHg, while feeding a methyl methacrylate solutioncontaining 5% by weight of hydroquinone from the top of the distillationtower at a flow rate of 40 g/hr. The temperatures at the uppermostportion and the lowermost portion of the distillation tower were 50° C.and 80° C., respectively. A steam containing methyl methacrylate wasrecovered from the top of the distillation tower and cooled by means ofa refrigerator to thereby obtain a condensate. The obtained condensatewas withdrawn at a rate of 500 g/hr.

8. Methyl Methacrylate (MMA) Purification Step

The condensate obtained in step 7 above was fed into a 70-stage platedistillation tower (inner diameter: 10 cm, height: 5 m) provided withsieve trays at the 35th plate as counted from the top thereof at a flowrate of 500 g/hr. An apparatus comprising a brine refrigerator forremoving a liquid under reduced pressure was provided at the top of thedistillation tower, and an apparatus which was controlled by means of alevel gauge and which comprised a refrigerator for withdrawing steamunder reduced pressure and cooling the withdrawn steam to thereby obtaina condensate, was provided at the bottom of the distillation tower. Thedistillation tower was continuously operated under conditions whereinthe reflux rate was 225 g/hr and the pressure at the uppermost portionof the distillation tower was 140 mmHg, while feeding a methylmethacrylate solution containing 5% by weight of hydroquinone from thetop of the distillation tower at a flow rate of 40 g/hr. Thetemperatures at the uppermost portion and the lowermost portion of thedistillation tower were 55° C. and 80° C., respectively. A steamcontaining methyl methacrylate was recovered from the top of thedistillation tower and cooled by means of a refrigerator, therebyobtaining a purified methyl methacrylate at a flow rate of 450 g/hr.

The thus obtained purified methyl methacrylate had an APHA value ofapproximately 3, and a methyl methacrylate polymer produced using themethyl methacrylate monomer had a YI value of 3.5. Therefore, the methylmethacrylate monomer and methyl methacrylate polymer produced using theoxide catalyst composition of the present invention had excellent APHAvalue and excellent YI value, respectively.

COMPARATIVE EXAMPLE 1

An oxide catalyst composition having a structure (in terms of atomicratios of constituent metallic elements, relative to twelve atoms of thetotal of molybdenum and tungsten) represented by the formula:Mo₁₂ Bi1.6Ce_(0.4)K_(0.1)Cs_(0.4)Fe_(1.5)CO_(8.0)was prepared as follows.

362 g of ammonium heptamolybdate was dissolved in 1,750 g of waterhaving a temperature of about 50° C., to thereby obtain an aqueoussolution (referred to as “solution A”). On the other hand, 133 g ofbismuth nitrate, 30.1 g of cerium nitrate, 1.72 g of potassium nitrate,103.9 g of iron nitrate, 402 g of cobalt nitrate and 13.4 g of cesiumnitrate were dissolved in 355 g of a 15% by weight aqueous nitric acidsolution, to thereby obtain a solution (referred to as “solution B”).Solutions A and B were mixed together while stirring for about 2 hours,thereby obtaining a slurry of the starting materials. The obtainedslurry was subjected to spray drying, to thereby obtain a dried,particulate catalyst composition precursor. The obtained dried,particulate catalyst composition precursor was calcined at 200° C. for 3hours, to thereby obtain a calcined catalyst composition precursor inthe form of a quasispherical particle. The obtained calcined catalystcomposition precursor was molded into a columnar tablet having adiameter of 5.0 mm and a height of 4 mm, and the tablet was subjected tofinal firing at 500° C. for 3 hours, thereby obtaining a final tabletedoxide catalyst composition.

Methacrolein was produced in the same manner as in Example 1 in order toevaluate the initial performances of the oxide catalyst composition. Theresults of the reaction were evaluated, and it was found that theconversion of isobutylene was 97.4%, the selectivity for methacroleinwas 86.5% and the selectivity for methacrylic acid was 2.4%. Theanalysis of the condensed gaseous product showed that the production ofdiacetyl was 3,500 ppm, the S1 value was 20, and the S2 value was 56.

Subsequently, a test under stringent conditions was performed in thesame manner as in Example 1 to evaluate the performances of the catalystcomposition under stringent conditions. It was found that the conversionof isobutylene was 97.4%, the selectivity for methacrolein was 86.3% andthe selectivity for methacrylic acid was 2.4%. The analysis of thecondensed gaseous product showed that the production of diacetyl was3,600 ppm, the S1 value was 21, and the S2 value was 56.

In addition, a methyl methacrylate monomer was produced in the samemanner as in Example 1 by the direct ML-to-MMA process by using theabove-mentioned oxide catalyst composition. The thus obtained methylmethacrylate monomer had an APHA value of 8, and a methyl methacrylatepolymer produced using the methyl methacrylate monomer had a YI value of12.5. Therefore, the APHA value and YI value of the produced methylmethacrylate monomer and methyl methacrylate polymer were poor, ascompared to those in the case of the use of the oxide catalystcomposition of the present invention.

EXAMPLE 2

In Example 2, oxide catalyst composition having the formulations asshown in Table 1 was prepared in substantially the same manner as inExample 1, except that the amounts of metallic elements were adjusted soas to comply with the formulation indicated in Table 1. In addition, thefinal firing of the catalyst was conducted at the temperature indicatedin Table 1.

The evaluation of the initial performances of the oxide catalystcomposition and the stringent condition test were performed in the samemanner as in Example 1. The initial performances of the catalystcomposition are shown in Table 2 and the performances of the catalystcomposition after the stringent condition test are shown in Table 3. Inaddition, a methyl methacrylate monomer was produced by the directML-to-MMA process in the same manner as in Example 1, and a methylmethacrylate polymer was produced using the thus produced methylmethacrylate monomer. The APHA value of the methyl methacrylate monomerand the YI value of the methyl methacrylate polymer are shown in Table4.

EXAMPLE 3

An oxide catalyst composition having a structure (in terms of atomicratios of constituent metallic elements, relative to twelve atoms of thetotal of molybdenum and tungsten) represented by the formula:Mo_(9.5)W_(2.5)Bi_(1.7)Ce_(0.4)K_(0.2)Cs_(0.2)Fe_(1.0)Co_(6.5)Ni_(1.0)Sb_(0.5)was prepared as follows.

103.6 g of ammonium paratungstate was dissolved in 1,900 g of waterhaving a temperature of about 60° C., and 267.1 g of ammoniumheptamolybdate was added thereto, thereby obtaining an aqueous solution.To the obtained aqueous solution was added 11.6 g of antimony trioxide,thereby obtaining an aqueous suspension containing molybdenum, tungstenand antimony. The obtained aqueous suspension was heated to 90° C. whilestirring, and then 50.0 g of a 30% by weight aqueous hydrogen peroxidesolution was slowly added thereto while stirring. The resultant mixtureturned to assume bright yellow while foaming, and the state of themixture became a solution. The stirring of the thus obtained aqueoussolution was further continued for about 30 minutes at 90° C. Then, thesolution was cooled to 50° C. and the solution was maintained at 50° C.(the thus obtained solution is referred to as “solution A”). On theother hand, 131.9 g of bismuth nitrate, 28.0 g of cerium nitrate, 3.21 gof potassium nitrate, 64.6 g of iron nitrate, 302.2 g of cobalt nitrate,46.3 g of nickel nitrate and 6.23 g of cesium nitrate were dissolved in280 g of a 15% by weight aqueous nitric acid solution, to thereby obtaina solution (referred to as “solution B”). Solutions A and B were mixedtogether while stirring for about 2 hours, thereby obtaining a slurry ofthe starting materials. The obtained slurry was subjected to spraydrying, to thereby obtain a dried, particulate catalyst compositionprecursor. The obtained dried, particulate catalyst compositionprecursor was calcined at 200° C. for 3 hours, to thereby obtain acalcined catalyst composition precursor in the form of quasisphericalparticles. The obtained calcined catalyst composition precursor wasmolded into a columnar tablet having a diameter of 5.0 mm and a heightof 4 mm, and the tablet was subjected to final firing at 520° C. for 3hours, thereby obtaining a final tableted oxide catalyst composition.

The evaluation of the initial performances of the catalyst compositionand the stringent condition test were performed in the same manner as inExample 1. The initial performances of the catalyst composition areshown in Table 2 and the performances of the catalyst composition afterthe stringent condition test are shown in Table 3. In addition, a methylmethacrylate monomer was produced by the direct ML-to-MMA process in thesame manner as in Example 1, and a methyl methacrylate polymer wasproduced using the thus produced methyl methacrylate monomer. The APHAvalue of the methyl methacrylate monomer and the YI value of the methylmethacrylate polymer are shown in Table 4.

EXAMPLES 4 TO 20

In Examples 4 to 20, oxide catalyst compositions having respectiveformulations as shown in Table 1 were prepared in substantially the samemanner as in Example 1 or Example 3, except that the sources of metallicelements and amounts thereof were selected so as to comply with therespective formulations indicated in Table 1. Specifically, the oxidecatalyst compositions not containing tungsten were prepared insubstantially the same manner as in Example 1, and the oxide catalystcompositions containing tungsten were prepared in substantially the samemanner as in Example 3. In addition, the final firing of the catalystswas conducted at the respective temperatures indicated in Table 1.

The evaluation of the initial performances of the oxide catalystcompositions and the stringent condition tests were performed in thesame manner as in Example 1. The initial performances of the catalystcompositions are shown in Table 2 and the performances of the catalystcompositions after the stringent condition test are shown in Table 3. Inaddition, methyl methacrylate monomers were produced by the directML-to-MMA process in the same manner as in Example 1, and methylmethacrylate polymers were produced using the respective methylmethacrylate monomers individually. The APHA values of the methylmethacrylate monomers and the YI values of the methyl methacrylatepolymers are shown in Table 4.

COMPARATIVE EXAMPLES 2 TO 16

In Comparative Examples 2 to 16, oxide catalyst compositions havingrespective formulations as shown in Table 1 were prepared insubstantially the same manner as in Comparative Example 1, Example 1 orExample 3, except that the sources of metallic elements and amountsthereof were selected so as to comply with the respective formulationsindicated in Table 1. Specifically, the oxide catalyst compositions notcontaining antimony were prepared in substantially the same manner as inComparative Example 1, the oxide catalyst compositions containingantimony, but not containing tungsten were prepared in substantially thesame manner as in Example 1, and the oxide catalyst compositionscontaining antimony and tungsten were prepared in substantially the samemanner as in Example 3. In addition, the final firing of the catalystswas conducted at the respective temperatures indicated in Table 1. Theresults of the evaluation of the initial performances of the oxidecatalyst compositions are shown in Table 2.

Each of the oxide catalyst compositions of Comparative Examples 3, 6, 7and 8, each of which exhibited a selectivity for methacrolein of 86.5%or more, was subjected to the stringent condition test in the samemanner as in Example 1. The performances of the catalyst compositionsafter the stringent condition test are shown in Table 3. In addition,methyl methacrylate monomers were produced by the direct ML-to-MMAprocess in the same manner as in Example 1, and methyl methacrylatepolymers were produced using the respective methyl methacrylatemonomers. The APHA values of the methyl methacrylate monomers and the YIvalues of the methyl methacrylate polymers are shown in Table 4.

TABLE 1 Formulation of the catalyst compositions and the temperatureused for final firing Temp. used Oxide catalyst composition for finalfiring Formula (I) (Mo + W)₁₂ Bi_(a) A_(b) B_(c) Fe_(d) X_(e) Sb_(f) (°C.) Ex. 1 Mo₁₂ Bi_(1.6) Ce_(0.4) K_(0.1) Cs_(0.4) Fe_(1.5) Co_(8.0)Sb_(0.7) 520 Ex. 2 Mo₁₂ Bi_(1.2) Ce_(0.6) K_(0.2) Cs_(0.1) Rb_(0.3)Fe_(2.1) Co_(6.0) Mg₂ Sb_(0.85) 510 Ex. 3 Mo_(9.5) W_(2.5) Bi_(1.7)Ce_(0.4) K_(0.2) Cs_(0.2) Fe_(1.0) Co_(6.5) Ni₁ Sb_(0.5) 520 Ex. 4 Mo₁₂Bi_(0.24) Yb_(0.06) K_(0.1) Cs_(0.3) Fe_(2.4) Co_(8.0) Sb_(0.8) 530 Ex.5 Mo_(11.6) W_(0.4) Bi_(0.3) Sm_(0.1) K_(1.0) Cs_(0.4) Fe_(2.1) Co_(7.0)Mg₂ Sb_(0.5) 510 Ex. 6 Mo₁₂ Bi_(0.4) Ce_(0.9) K_(0.4) Cs_(0.15) Fe_(1.9)Co_(8.0) Sb_(0.7) 510 Ex. 7 Mo₁₁ W₁ Bi_(0.55) La_(0.2) K_(0.2) Cs_(0.4)Fe_(2.6) Co_(8.0) Sb_(0.3) 540 Ex. 8 Mo_(11.7) W_(0.3) Bi_(0.55)Ce_(0.2) K_(0.1) Cs_(0.6) Fe_(2.2) Co_(5.6) Ni_(2.4) Sb_(1.9) 510 Ex. 9Mo₁₂ Bi_(2.0) Ce_(0.6) K_(0.5) Cs_(0.6) Fe_(4.0) Co_(6.0) Mg_(2.0)Ni_(1.0) Sb_(2.5) 550 Ex. 10 Mo₁₂ Bi_(1.6) Y_(0.4) K_(0.1) Rb_(0.4)Fe_(1.5) Co_(8.0) Sb_(0.7) 510 Ex. 11 Mo₁₀ W₂ Bi_(1.6) Pr_(0.4) Cs_(0.4)Rb_(0.2) Fe_(1.5) Co_(8.0) Sb_(0.7) 510 Ex. 12 Mo₁₂ Bi_(0.8) Sm_(0.9)K_(0.15) Cs_(0.35) Fe_(1.9) Co_(8.0) Sb_(1.4) 520 Ex. 13 Mo_(11.9)W_(0.1) Bi_(0.8) Nd_(0.5) Pr_(0.4) K_(0.15) Cs_(0.35) Fe_(1.9) Co_(8.0)Sb_(1.5) 520 Ex. 14 Mo₁₂ Bi_(0.8) Ce_(0.9) K_(0.1) Cs_(0.1) Fe_(2.6)Co_(8.0) Sb_(0.8) 520 Ex. 15 Mo₁₂ Bi_(0.6) Ce_(0.2) K_(0.1) Cs_(0.4)Fe_(1.9) Co₄ Mg_(4.0) Sb_(1.5) 510 Ex. 16 Mo₁₂ Bi_(0.55) Ce_(0.15)K_(0.1) Cs_(0.4) Fe_(2.3) Co_(8.0) Sb_(0.85) 530 Ex. 17 Mo₁₂ Bi_(0.55)Ce_(0.15) K_(0.1) Cs_(0.4) Fe_(1.3) Co_(5.0) Mg_(3.0) Sb_(0.85) 510 Ex.18 Mo₁₁ W₁ Bi_(0.55) Ce_(0.15) K_(0.1) Cs_(0.4) Fe_(2.3) Co_(7.0)Sb_(1.5) 530 Ex. 19 Mo₁₂ Bi_(0.9) Ce_(0.7) Rb_(0.5) Cs_(0.2) Fe_(3.1)Co_(7.0) Sb_(1.6) 530 Ex. 20 Mo₁₂ Bi₁ Ce_(0.9) Rb_(0.5) Fe_(3.5)Co_(11.0) Sb_(1.8) 540 Compara. Ex. 1 Mo₁₂ Bi_(1.6) Ce_(0.4) K_(0.1)Cs_(0.4) Fe_(1.5) Co_(8.0) 500 Compara. Ex. 2 Mo₈ W₄ Bi_(1.6) Ce_(0.2)K_(0.1) Cs_(0.4) Fe_(1.5) Ni_(8.0) 510 Compara. Ex. 3 Mo₁₂ Bi_(0.16)Y_(0.04) K_(0.1) Cs_(0.3) Fe_(2.4) Co_(8.0) Sb_(0.8) 510 Compara. Ex. 4Mo₁₁ W₁ Bi_(0.16) Sm_(0.04) K_(1.3) Cs_(0.2) Fe_(2.1) Co_(7.0) Mg_(2.0)Sb_(0.5) 510 Compara. Ex. 5 Mo_(11.5) W_(0.5) Bi_(0.3) Ce_(1.2) K_(0.15)Cs_(0.35) Fe_(1.9) Co_(8.0) Sb_(0.7) 510 Compara. Ex. 6 Mo₁₂ Bi_(1.5)La_(0.03) K_(0.15) Cs_(0.35) Fe_(1.9) Co_(8.0) Sb_(0.7) 510 Compara. Ex.7 Mo₁₂ Bi_(0.55) Sm_(0.2) K_(0.2) Cs_(0.4) Fe_(2.6) Co_(8.0) Sb_(0.1)520 Compara. Ex. 8 Mo₉ W₃ Bi_(0.55) Nd_(0.2) K_(0.2) Cs_(0.4) Fe_(2.5)Co_(8.0) Sb_(2.4) 520 Compara. Ex. 9 Mo₁₂ Bi_(2.0) Ce_(0.6) K_(0.5)Cs_(0.5) Fe_(5.0) Co₁₀ Sb_(2.5) 530 Compara. Ex. 10 Mo₁₁ W₁ Bi_(1.6)Y_(0.4) K_(0.2) Cs_(0.5) Fe_(0.5) Co_(8.0) Mg_(2.0) Sb_(0.6) 510Compara. Ex. 11 Mo₁₂ Bi_(1.6) Pr_(0.4) K_(0.1) Cs_(0.4) Fe_(1.5)Co_(6.0) Ni_(3.0) Sb_(0.7) 520 Compara. Ex. 12 Mo₁₂ Bi_(0.6) Ce_(0.2)K_(0.1) Cs_(0.1) Fe_(2.6) Co₂ Mg_(6.0) Sb_(0.8) 510 Compara. Ex. 13 Mo₁₂Bi_(2.0) Ce_(0.6) K_(0.5) Cs_(0.6) Fe_(4.0) Co_(6.0) Mg_(2.0) Ni_(1.0)Sb_(3.0) 550 Compara. Ex. 14 Mo₁₂ Bi_(1.6) Ce_(0.4) K_(0.1) Rb_(0.4)Fe_(1.5) Ni_(8.0) Sb_(0.7) 520 Compara. Ex. 15 Mo₁₂ Bi_(1.6) Ce_(0.4)K_(0.1) Rb_(0.4) Fe_(1.5) Co_(2.0) Mg_(2.0) Ni_(6.0) Sb_(0.7) 520Compara. Ex. 16 Mo₁₂ Bi_(1.6) Ce_(0.4) K_(0.1) Ce_(0.4) Fe_(1.5)Co_(10.0) Mg_(3.0) Sb_(0.7) 540

TABLE 2 Evaluation of the initial performances of the oxide catalystcomposition Conversion of Selectivity for Selectivity for Production ofIntensity Intensity isobutylene methacrolein methacrylic acid diacetylof R1* of R2* (%) (%) (%) (ppm) (S1) (S2) Ex. 1 97.8 88.3 2.4 500 10 52Ex. 2 97.5 88.5 2.3 460 8 53 Ex. 3 98.0 88.1 2.6 510 12 49 Ex. 4 97.087.9 2.1 720 6 41 Ex. 5 97.2 88.0 2.2 640 7 45 Ex. 6 97.4 88.1 2.2 610 949 Ex. 7 97.7 88.6 2.4 400 10 50 Ex. 8 97.9 88.5 2.4 350 12 48 Ex. 996.9 88.2 2.1 620 8 42 Ex. 10 97.9 87.9 2.0 560 11 41 Ex. 11 97.8 88.12.2 540 13 48 Ex. 12 97.6 88.4 2.5 490 7 39 Ex. 13 97.5 88.3 2.4 550 937 Ex. 14 97.6 88.0 2.6 400 11 44 Ex. 15 97.7 88.2 2.3 450 10 41 Ex. 1697.6 88.3 2.3 380 8 43 Ex. 17 97.0 88.0 2.2 430 11 46 Ex. 18 97.3 87.92.4 420 10 48 Ex. 19 97.4 88.1 2.3 470 9 39 Ex. 20 97.1 87.9 2.3 490 1246 Compara. Ex. 1 97.4 86.5 2.4 3500 20 56 Compara. Ex. 2 97.1 84.4 2.52460 18 55 Compara. Ex. 3 97.2 86.6 2.3 1810 22 53 Compara. Ex. 4 96.886.1 2.1 1560 27 59 Compara. Ex. 5 97.1 85.8 2.3 1710 26 56 Compara. Ex.6 97.2 86.5 2.1 1660 25 55 Compara. Ex. 7 97.3 86.6 2.2 2440 28 54Compara. Ex. 8 97.5 86.6 2.3 2830 21 53 Compara. Ex. 9 98.2 84.0 2.53130 19 51 Compara. Ex. 10 95.4 86.0 2.1 4640 32 60 Compara. Ex. 11 97.285.5 2.2 1350 18 51 Compara. Ex. 12 95.2 86.0 2.1 950 19 51 Compara. Ex.13 97.1 84.5 2.6 1030 17 49 Compara. Ex. 14 97.2 84.7 2.5 2390 17 53Compara. Ex. 15 96.9 85.1 2.4 2160 20 51 Compara. Ex. 16 97.5 84.9 2.32740 19 48 *S1 and S2 (which are, respectively, the percentages of theareas of R1 and R2, each based on the peak area of diacetyl) were usedas indices for the intensities of R1 and R2.

TABLE 3 Evaluation of the oxide catalyst composition after the stringentcondition test Conversion of Selectivity for Selectivity for Productionof Intensity Intensity isobutylene methacrolein methacrylic aciddiacetyl of R1* of R2* (%) (%) (%) (ppm) (S1) (S2) Ex. 1 97.8 88.3 2.4490 10 51 Ex. 2 97.5 88.5 2.3 400 7 50 Ex. 3 98.0 88.1 2.6 440 11 48 Ex.4 97.0 88.0 2.1 630 6 39 Ex. 5 97.2 88.1 2.2 640 7 42 Ex. 6 97.4 88.12.2 510 8 47 Ex. 7 97.7 88.6 2.4 350 9 49 Ex. 8 97.9 88.6 2.4 310 12 46Ex. 9 96.9 88.2 2.1 540 7 37 Ex. 10 97.9 88.0 2.0 480 9 37 Ex. 11 97.887.9 2.2 490 12 46 Ex. 12 97.6 88.4 2.5 440 6 37 Ex. 13 97.5 88.3 2.4500 8 35 Ex. 14 97.6 88.0 2.6 390 11 42 Ex. 15 97.7 88.3 2.3 440 9 41Ex. 16 97.6 88.3 2.3 370 9 44 Ex. 17 97.0 88.0 2.2 430 11 46 Ex. 18 97.387.9 2.4 410 10 49 Ex. 19 97.4 88.1 2.3 470 9 39 Ex. 20 97.1 87.9 2.3490 12 45 Compara. Ex. 1 97.4 86.3 2.4 3600 21 56 Compara. Ex. 3 97.286.6 2.2 1830 24 54 Compara. Ex. 6 96.3 86.4 2.1 1680 27 56 Compara. Ex.7 97.3 86.5 2.2 2480 28 55 Compara. Ex. 8 97.4 86.6 2.2 2850 22 53 *S1and S2 (which are, respectively, the percentages of the areas of R1 andR2, each based on the peak area of diacetyl) were used as indices forthe intensities of R1 and R2.

TABLE 4 APHA values of methyl methacrylate monomers and YI values ofmethyl methacrylate polymers APHA of YI of methyl methacrylate methylmethacrylate monomer polymer Ex. 1 3 3.5 Ex. 2 3 3.2 Ex. 3 3 3.5 Ex. 4 55.0 Ex. 5 4 4.5 Ex. 6 4 4.4 Ex. 7 3 3.1 Ex. 8 2 2.7 Ex. 9 4 4.4 Ex. 10 33.9 Ex. 11 3 3.7 Ex. 12 3 3.5 Ex. 13 3 3.7 Ex. 14 3 3.0 Ex. 15 3 3.2 Ex.16 2 2.9 Ex. 17 3 3.2 Ex. 18 3 3.2 Ex. 19 3 3.3 Ex. 20 3 3.3 Compara.Ex. 1 8 12.5 Compara. Ex. 2 8 12.0 Compara. Ex. 3 7 11.8 Compara. Ex. 47 11.5 Compara. Ex. 5 7 11.7 Compara. Ex. 6 7 11.7 Compara. Ex. 7 8 12.0Compara. Ex. 8 8 12.2 Compara. Ex. 9 8 12.3 Compara. Ex. 10 9 13.4Compara. Ex. 11 7 11.3 Compara. Ex. 12 6 10.5 Compara. Ex. 13 7 11.0Compara. Ex. 14 8 12.0 Compara. Ex. 15 8 12.0 Compara. Ex. 16 8 12.2

As apparent from the results shown in Tables 2 to 4 above, in the caseof the production of methacrolein in the Examples, not only the amountof by-produced diacetyl, but also the amounts of the impurities to whichR1 and R2 are ascribed were low. Further, when methyl methacrylate wasproduced by the direct ML-to-MMA process performed using the oxidecatalyst composition of the present invention as a first stage catalyst,the produced methyl methacrylate monomer suffered substantially nodiscoloration. In addition, the polymer obtained by polymerizing themethyl methacrylate monomer also suffered substantially nodiscoloration. As apparent from Table 3 above, even in the stringentcondition test in which methacrolein was produced at a high temperature,each of the oxide catalyst compositions produced in the Examplesexhibited a high selectivity for methacrolein, which was substantiallythe same as the initial performances, and also the by-production ofdiacetyl and the impurities to which R1 and R2 are ascribed was low.Therefore, as compared to the conventional oxide catalyst compositions,the oxide catalyst composition of the present invention by-produces onlysmall amounts of impurities and exhibits excellent properties withrespect to thermal stability and reduction resistance.

INDUSTRIAL APPLICABILITY

The oxide catalyst composition of the present invention exhibits notonly a prolonged catalyst life due to its excellent properties withrespect to thermal stability and reduction resistance, but alsoexcellent selectivity for the desired product. By the use of the oxidecatalyst composition of the present invention for producing methacroleinor a mixture of methacrolein and methacrylic acid, it becomes possibleto stably produce the desired product for a long time while holding downthe amount of by-produced impurities, e.g. diacetyl. The producedmethacrolein or mixture of methacrolein and methacrylic acid has lowcontents of the by-produced impurities, e.g. diacetyl, and suchmethacrolein or mixture of methacrolein and methacrylic acid is veryadvantageous as a raw material for producing methyl methacrylate havingexcellent transparency. A methyl methacrylate polymer having excellenttransparency, which can be obtained by polymerizing such highlytransparent methyl methacrylate monomer, can be advantageously used as asubstitute for glass and quartz in application fields requiring hightransparency, such as optical fibers, light guide plates and the like;thus, such highly transparent methyl methacrylate polymer has very highcommercial value.

1. An oxide catalyst composition for use in producing methacrolein or amixture of methacrolein and methacrylic acid by reacting at least onemember selected from the group consisting of isobutylene and t-butanolwith a molecular oxygen-containing gas, said oxide catalyst compositionbeing represented by the following formula (I):(Mo+W)₁₂Bi_(a)A_(b)B_(c)Fe_(d)X_(e)Sb_(f)O_(g)  (I) wherein: A is atleast one member selected from the group consisting of lanthanum,cerium, praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium and|yttrium; B is at least one member selected from the group consisting ofpotassium, rubidium and cesium; X is cobalt solely, or a mixture ofcobalt and at least one member selected from the group consisting ofmagnesium and nickel; wherein the number of molybdenum (Mo) atoms is inthe range of from more than 9 to 12, and the number of tungsten (W)atoms is in the range of from 0 to less than 3, each relative to twelveatoms of the total of molybdenum (Mo) and tungsten (W); and a, b, c, d,e, f and g are, respectively, the atomic ratios of bismuth (Bi), A, B,iron (Fe), X, antimony (Sb) and oxygen (O), relative to twelve atoms ofthe total of molybdenum (Mo) and tungsten (W),  wherein 0<a≦8, 0<b≦8,0<c≦3, 0.2<d<5, 1≦e≦12, 0.1<f<3, and g is the number of oxygen atomsrequired to satisfy the valence requirements of the other elementspresent; and wherein a, b, c, d and f satisfy the requirements of thefollowing formulae:0.02<b/(a+b+c)<0.6,0<c/(a+b+c)≦0.9,0.01<d/(a+b+d)≦0.9, and0.1<d−f<2.5.
 2. The oxide catalyst composition according to claim 1,wherein, in said mixture X in formula (I), the atomic ratio of cobalt tothe total of cobalt, magnesium and nickel is 0.5 or more, wherein, whensaid mixture X in formula (I) contains magnesium, the atomic ratio ofmagnesium to the total of cobalt, magnesium and nickel in said mixture Xis 0.5 or less, and wherein, when said mixture X in formula (I) containsnickel, the atomic ratio of nickel to the total of cobalt, magnesium andnickel in said mixture X is less than 0.33.
 3. The oxide catalystcomposition according to claim 1 or 2, wherein a, b and c in formula (I)satisfy the requirements of the formula: 0.05<b/(a+b+c)<0.5.
 4. Theoxide catalyst composition according to claim 1 or 2, wherein a, b and cin formula (I) satisfy the requirements of the formula:0.1<c/(a+b+c)<0.8.
 5. The oxide catalyst composition according to claim1 or 2, wherein a, b, d and f in formula (I) satisfy the requirements ofthe formulae:0.2<d/(a+b+d)<0.9 and 0.3≦d−f≦2.3.
 6. A method for producingmethacrolein or a mixture of methacrolein and methacrylic acid, whichcomprises reacting at least one member selected from the groupconsisting of isobutylene and t-butanol with a molecularoxygen|-containing gas in the presence of the oxide catalyst compositionof claim 1 or 2, thereby obtaining methacrolein or a mixture ofmethacrolein and methacrylic acid.
 7. A method for producing methylmethacrylate, which comprises: (i) reacting at least one member selectedfrom the group consisting of isobutylene and t-butanol with a molecular|oxygen-containing gas in the presence of the oxide catalyst compositionof claim 1 or 2, to thereby obtain methacrolein; (ii) subjecting theobtained methacrolein to a gaseous phase catalytic oxidation reactionwith molecular oxygen, to thereby obtain methacrylic acid; and (iii)subjecting the obtained methacrylic acid to an |esterification withmethanol, thereby obtaining methyl methacrylate.
 8. A method forproducing methyl methacrylate, which comprises: (i) reacting at leastone member selected from the group consisting of isobutylene andt-butanol with a molecular |oxygen-containing gas in the presence of theoxide catalyst composition of claim 1 or 2, to thereby obtainmethacrolein; and (ii) reacting the obtained methacrolein with methanol,thereby obtaining methyl methacrylate.